The present invention relates to an image-recording composition, particular to a pressure-sensitive, heat-sensitive image-recording composition, which rapidly develops a color even at a low temperature of 50 to 90xc2x0 C. to provide a clear image in a short time, thereby being useful for a high-resolution printer. The present invention also relates to an image-recording sheet using the image-recording composition.
Conventionally, high-resolution image-recording has been achieved by pressure-sensitive, heat-sensitive image-recording methods using a microcapsule composed of a shell wall enclosing a color-producing agent such as a leuco-dye, etc. The color-producing agent reacts with a color-developing agent to exhibit a color. A pressure-sensitive, heat-sensitive image-recording sheet produced by applying the microcapsule and the color-developing agent to a sheet is used in a typical pressure-sensitive, heat-sensitive image-recording methods. The image-recording sheet is heated under a pressure by a thermal head, etc., whereby the microcapsule is broken to release the color-producing agent. Then, the color-producing agent reacts with the color-developing agent to provide an image on the sheet.
In the pressure-sensitive, heat-sensitive image-recording methods, the shell wall of the microcapsule is controlled with respect to the thickness and material, such that the microcapsule is broken only by heating at a predetermined breaking temperature under a predetermined breaking pressure. Thus, a high resolution multi-color image can be obtained by these methods if several kinds of microcapsules, which are broken at a different breaking temperature under a different breaking pressure to exhibit a different color such as cyan, magenta, yellow, etc., are applied to the sheet.
Although the breaking temperature of the microcapsule is generally 100xc2x0 C. or more, preferably 120xc2x0 C. or more from the viewpoint of stability and reliability of the image-recording sheet, the breaking temperature and the breaking pressure of each microcapsule should be controlled to selectively break only one desired kind of microcapsule in the case of using the several kinds of microcapsules. For example, when three kinds of microcapsules are used for the image-recording sheet as shown in FIG. 1, a cyan-exhibiting region A, a magenta-exhibiting region B and a yellow-exhibiting region C must not be overlapped. Therefore, in this case, at least one kind of microcapsule preferably has a low breaking temperature.
However, even if the microcapsule has a low breaking temperature, the leuco-dye is often poor in the color-developing reaction rate, thereby requiring long period of time, several hours, in developing a sufficiently deep color. Particularly, yellow color developed by the leuco-dye is extremely low in depth, resulting in a multi-color image poor in color balance.
An object of the present invention is to provide an image-recording composition that rapidly develops a color not depending on kind of leuco-dye to provide a clear image in a short time, and an image-recording sheet using the image-recording composition.
As a result of intense research in view of the above object, the inventors have found that an image-recording composition comprising: a microcapsule enclosing a leuco-dye; a phenol color-developing agent; and a reaction-accelerating agent catalyzing a color-developing reaction between the leuco-dye and the phenol color-developing agent, where an organic compound having at least two carboxyl groups is used as the reaction-accelerating agent, can rapidly develop a color to be made practicable. The present invention has been accomplished by the finding.
The reaction-accelerating agent used in the present invention is not such that acts to reduce color-developing temperature as a sensitizer. The reaction-accelerating agent does not affect the color-developing agent before breaking the microcapsule, and it accelerates the color-developing reaction so that the leuco-dye reacts with the color-developing agent to exhibit a color immediately after breaking the microcapsule. Every leuco-dye including such as poor in the color-developing reaction rate can rapidly develop a color by the reaction-accelerating agent in the present invention. Therefore, in the case of using a plurality of leuco-dyes, the image-recording composition of the present invention can provide a multi-color image excellent in color balance.
Generally, a leuco-dye high in the color-developing reaction rate is also high in the color-discharging rate. Thus, to rapidly develop a color and not to discharge the color for a long time, a plurality of leuco-dyes including a dye high in the color-developing reaction rate, a dye poor in the color-developing reaction rate, a dye balancing the developed color, etc. have been used even in the mono-color image-recording sheet. According to the present invention, the color-developing reaction rate is controlled by the reaction-accelerating agent, whereby the number of the leuco-dyes and the amount of the color-developing agent is reduced to reduce costs.
In the image-recording composition of the present invention, the ratio of [the number of the carboxyl groups of the reaction-accelerating agent]/[the number of carbon atom(s) of the reaction-accelerating agent other than carbon atoms forming the carboxyl groups] is preferably 0.2 or more. The ratio is hereinafter referred to as xe2x80x9ccarboxyl group-contentxe2x80x9d. The reaction-accelerating agent having a high carboxyl group-content sufficiently accelerates the color-developing reaction.